mu 3.12.0

A delegate that ends without an answer leaves its evidence. Timeout,
API error, empty finish, dead pane or cancel: the follow-up names the
delegate's pi session file and copies its last message so far to
~/.cache/mu/delegate-answers/.

Upgrade with npm i -g @mu-crew/mu@3.12.0, then /reload in running
pi sessions to pick up the extension. No schema change. The session
file is reported by the delegate's own extension, so delegates need
3.12.0 too; an older one still gets the mu agent read pointer.

Added

• Delegates that end without an answer leave evidence. A
  mu_delegate follow-up for an API error, an empty finish, a
  timeout or a dead pane, and the mu_delegate_cancel result, now
  name the delegate's pi session file and copy its last message so far
  to ~/.cache/mu/delegate-answers/<ts>_<name>.partial.md. Before, the
  only pointer was mu agent read, which stops working once the pane
  is gone. Quitting pi or /reload lists each running delegate's
  session file too.
• The control socket's status reports sessionFile, so the reply
  to mu agent spawn --send / mu agent send carries it before any
  run settles.